An example of an application area for holding brakes is numerically controlled machine tools. For each of the axes of such a machine tool, a drive is provided, the motor of which is controlled via a closed-loop control structure, including, for example, positional controllers, speed controllers and current controllers. In this example, positional values are input by a numerical control and are then converted by the control structure into drive signals for the power circuitry of the motor. In the control parameter setting, the decision is made as to how precisely the drive, including the motor, power circuitry, and control structure, realizes the inputs from the numerical control. Thus, in a PI controller, it is, above all, the gain factors in the proportional and integral parts of the controller that may be important. They determine the intensity of the system's reaction to a deviation from the particular setpoint value.
The drive of a machine tool is, in fact, able to hold an axis in a preset position. However, this is no longer effective when the drive is de-energized, for example due to a power failure. For that reason, on machine tools, it is conventional to provide mechanical holding brakes as well, which are used in the currentless state. To release these holding brakes, a current must flow, for example, through an electromagnet. The need arises, above all, for suspension axes to be stopped via such holding brakes, since without drive control, a movement of the axis can be caused by the gravity of the suspended load.
If it is intended to stop an axis of a machine tool, then it may be an important consideration that the time of the axis-drive control overlap with the time in which the holding brake has already been engaged. Otherwise, one risks a short time span in which the axis is neither stopped by the drive nor by a holding brake.
The control parameters of a drive are optimized for an operation without an engaged holding brake. In a machine tool, in particular, the parameters are often adjusted in a manner which will allow the path deviations of a tool to be corrected very aggressively, in order to keep following errors or servo lag to a minimum. However, the conditions change completely when the holding brake is engaged, since the motor of the drive now suddenly experiences a substantially higher load. This leads to undesirable oscillations in the drive, which can be perceived by a whistling of the motor, for example. These oscillations are not only unpleasant for the operator, they also subject the holding brake to increased wear. It can even happen that the holding brake breaks loose, since the sliding or dynamic friction of a holding brake is distinctly less than the static friction.
Japanese Patent Publication No. 2000-47732 describes, when releasing a holding brake, to switch over between different gain factors in the control loop of a servo drive. By applying a high gain factor immediately after the holding brake is released and by switching over to a lower gain factor some time later, on the one hand, an axis should be prevented from falling and, on the other hand, oscillations in normal operation should be avoided. To avoid oscillations given a simultaneously active brake and active control, however, this procedure is not goal-directed when the brake is engaged, since it is precisely in this case that high gain factors lead to vibrations.
If, as a general principle, the control parameters are selected such that no oscillations occur even when the holding brake is engaged, then this has a negative effect on the quality of the control loop. The same applies to the use of filters in the control loop which avoid such oscillations, even if they are also active when the holding brake is not activated.
It is an object of the present invention to provide a method for actuating a holding brake which may enable oscillations to be avoided or greatly reduced, given a simultaneously active holding brake and active control.